Sound attenuating device



Original Filed April 50, 1934 5 a 9 M 07 rl/ n r L 9 F M w w w W 3% i y 5 w 3 w m; M

INVENTOR 04 mm 5. flay/m:- BY

@-W a M ATTORNEYS M v C Patented Oct. 15, 1935 course stares 2,01%?45 SOUND ATTENIIATING DEVICE Roland it. Become, H ford, Conn" assignor to The Maxim Silencer company, Hartford, Conn, a corporation of Connecticut @i'lglnal application Anvil so, 1934, Serial No.' 12$,d85. Divided and this application December 14, 1934, Serial No. 751,551

1c Claims. (cl. lei-0.5

The present invention relates to sound attenuating devices of the reactive type wherein one. or more acoustic sidebranches capable of showing resonance phenomena are coupled acoustically 5 to a main conducting channel or other enclosure wherein may occur sound waves which'are to be attenuated. A primary purpose or the inven- 'tion is to provide acoustic sidebranches having novel characteristics, and to show how the acoustic characteristics of these sidebranches may be used in connection with the design ofsound wave attenuating apparatus. A further purpose of the invention is to show how these character-.

istics may be applied to the design of silencers for use in connection with the exhausts or intakes of internal combustion engines, air compressors, blowers and the like. Further purposes and objects of the invention will be disclosed as the specification proceeds.

The acoustic sidebranches which are the main subject of this invention are designed and used on the basis that progressive change of phase as a function of distance occurs therein. For the purposes of the invention they will be referred to as linear sidebranches. As an example of a sound wave attenuating device employing linear sidebranches having progressive change of phase therein, reference is made to Bourne, United States Patent No. 1,910,672, May 23, 1933.

There are in general, two types of linear side. branches, i. e., those open at both ends, called open linear sidebranchesand those closed at one end, called closed? linear sidebranches.

When acoustic sidebranches are used in connection with silencers 'for internal combustion engincs andthe like, the use of an open side-.

branch may not be feasible since both exl'laust gas and sound can leave through the open end. The open type sidebranch possesses certain acoustic properties, however, that make it desirable from an acbustic point of view. It is well known that a cylindrical tube of uniform cross sectional area, open at both ends, is substantially one-half wave length long for its fundamental natural period or frequency and also resonates to all substantially harmonic overtones thereof,

both even and odd, whereas the closed tube of uniform cross sectional area is approximately one-quarter wave length long ior its'iundamentel frequency and respondsv to odd harmonics thereof only. I

Where reactive sidebranches are used as attenuating means for sound waves associated with exhaust and intakes of internal combustion engines and the like, it is desirable to oiler attenuation not only to the fundamental frequency oi the sound, but. also all its overtones. It is obvious that an acoustic sidebranch which will respend to and attenuate a full series oi harmonic tones and be entirely closed except for the point 5 at which it is coupled to the sound-bearing enclosure or conduit is very desirable. Heretofore, it has been impossible to combine the advantages of a closed linear sidebranch with one showing a response to a full series of harmonic over- 1o tones, both even and odd.

I have discovered that a sidebranch in the form of a. complete cone, open to the main sound chan- 1 nel at its large end and of course closed 0d at its pointed end, will respond. to-and attenuate s a complete series of harmonic overtones which are substantially multiples of the lowest frequency for which it resonates. In one aspect of the invention I employ such complete cones as closed linear sidebranches in an acoustic silenc- 2o ing device, these sidebranches responding to and attenuating in, the main channel or enclosure to which they are acoustically coupled, bands of frequencies, more or less wide, corresponding to and dependent-upon the fundamental frequency 5 of the conical sldebranches and all substantially integral multiples thereof, both even and odd. In. another aspect of the invention I employ as sidebranches, in an acoustic silencing device, structures having a cross sectional area decreasing g with-the distance irom their junction point with the main channel, suddciently slender to give substantial attenuation to at least one consecutive overtone of their fundamental frequency, and having an opening into the main channel 35 large enough to make the sidebranch operate mainly as a linear acoustic element.

Referring to the drawing,

Fig. 1 shows a truncated cone with open base, acoustically coupled to a main conducting chan- 40 nel;

Fig. 2 shows acomplete circular cone, continued to the apex, acoustically coupled at its base to a main conducting channel;

Fig. 3 shows in diagrammatic form a sound attenuating device embodying a single complete cone open at its louse;

Fig. 4. is a'section on line L-l of Fig. 3;

Fig. 5 shows a sound wave attenuating device embodying a conical and a closed cylindrical side- 59 branch; and

Fig. c shows theoretical operating characteristics of the device illustrated in Fig. 3.

This application is a division of my prior application Serial No. 723,985, filed April 30, 1934. so

In order to understand the operation of the devices of the invention it is useful to make use of impedance relations, since the operation of sidebranches in general can be completely expressed in terms of the acoustic impedance looking into the branch.

For a truncated cone such as is shown in Fig. 1, the impedance per unit area at the base, or point of coupling to the main channel III, in terms of the dimensions of the cone ll having a partition or header 42 a distance 21 from the apex and a slant length of zen-x1 may be mathematically shown to be i tan g 0 In the above equation the symbols have the following signiflcances, which hold true also for the further equations below with the changes and additions there noted:

Z.=acoustic impedance looking into the sidebranch.

j=frequency of sound wave.

C=ve1ocity of sound in the medium.

P=mean pressure of the sound transmitting medium.

=ratio of the specific heat of the medium at constant pressure to that at constant volume or. for waves of large amplitude, a function of this ratio.

=density of the medium.

zi=slant distance from the apex of the cone to its closed end (see Fig. 1).

12=slant distance from the apex of the cone to its junctionwith the main conducting channel, allowing for any necessary end correction.

It may be noted, in order to permit comparison with formulas for other tpes of silencing units given by different author that In Fig. 1 the conical sidebranch is connected to the main channel at its larger end, and diminishes in cross sectional area in a direction away from the-main channel. In the case of a true cone this change in cross sectional area is a simple function of the distance, while for certain sidebranches to be described below, the area depends upon the distance from the channel in a somewhat more complex manner. It should be noted particularly that the sidebranch decreases in area in a direction away from the main channel, as the performance is entirely din'erent'if the cone is turned with its small end toward the channel. Aswillnnwbesh Equation (1) maybeused as the basis for determining many of the acoustical characteristics of either truncated or complete 0015:: connected to the channel at their large en 1 P, uLo 1 whence where'n is any positive integer, either even or odd. Therefore L =n a result identical to that applying to a cylindrical tube open at both ends commonly called an "open" tube.

We now have an acoustical resonating device,

I completely closed except for an opening whereby it may be acoustically coupled to a main conducting channel and capable of being used to attenuate certain groups of sound frequencies comprising a fundamental group and all multiples thereof, both even and odd. The use of the term group rather than frequency is justified by the fact that resonating devices of the acoustic type exert their influence not only upon the theoretical single frequencies of resonance but also upon neighboring frequencies. The exact nature of the behavior of this type of resonating device under various conditions will be disclosed as the specification proceeds, and the advantages of certain specific arrangements will be shown.

In order to show the attenuating effect of a complete, cone acoustically coupled to a main conducting channel upon sound waves therein,

it is proper to first investigate the performance of a single such sidebranch, coupled to a relatively long main conducting channel. The attenuation in decibels may be shown to be I 2 Ndb=1 0 log [l+ (cot i where 81' is the area of the base of the cone.

S1 is the area of the main conducting channel.

A plot of Equation (4) is shown in Fig. 8, for the condition that Sz/S1=4.

In order to obtain a larger area for the base of the cone than exists in the main conducting channel, the cone may be disposed within a cas-. ing after the manner shown in Fig. 3. In this embodiment, the cone 45 is supported within the casing 48 in any convenient manner making due allowance for the passage of gas between the base of the cone and the casing and between the end of the cone and the header 41. The speciflc supporting construction vis not shown in detail as it is not necessary for a complete understanding of the invention. It is seen that the openbaleoftheconeislocateddirectlyoppositc to and closely adjacent the opening in the header 4? leading into the channel 48 which forms part of the main conducting channel through the device. In this particular embodimentof the invention, the space between the outside of the cone and the inside of the shell or casing is utilized to the purpose of attenuating sounds of relatively high frequency, such as bang and hiss noises usually associated with internal combustion engine exhausts. I have shown a simple baffle plate system 49 to represent such a high frequency attenuating means. Many arrangements of passageways, partitions, etc. may be used to advantage in this regard. The apex of the cone may be supported either by brackets or by a partition 50 extending from the casing to the cone and having therein holes 5i for the passage of the gas therethrough. It is theoretically preferable, in this and other embodiments, that the sidebranch retain its conical shape up to the point of coupling to the main channel. In

some cases, however, it may be desirable for constructional reasons to depart from the normal conical shape adjacent the zone of coupling. The departure from the theoretical response occa-= sioned by this slight change in shape will in most cases be of no practical consequence.

Referring again to Fig. 6 it is seen that maxi= mum attenuation occurs at values of =m Zr, 31, in, etc.

a tube would have an attenuation peak at zero frequency. It is to be noted that the conical sidebranch does not oifer attenuation to zero frequency, the attenuation decreasing continuously from a high value at to zero at as shown. The points of zero attenuation are not midway between the points oi maximum attenuation, as they are in the case of the open cylin drical sidebranch, out are displaced slightly therefrom, the displacement becoming less as the frequency increases. It can be shown, by a con sideration of Equation (4) that the condition for zero attenuation is given by the relation The first few roots of this equation are known to be It is seen that these successive values show a decreasing difierence from the midway points,

namely 1.51r, 3.51, etc. approaching quencies, of the device for which the curve shows operating characteristics, it is frequently desirable to make S: as large compared with Si as is commercially feasible with proper regard for the limitations imposed by acoustic theory.

suitably mounted and disposed within a casing 92 5 having a centrallydisposed inlet connection 93 and an eccentrically disposed outlet connection '94. The cone 9% has a simple bafile system 95 attached thereto. after the manner of the device shown in Fig. 3. This arrangement of the inte' w rior of the device permits the use of water ad-= mixed with the exhaust gas and the automatic draining thereof when the silencer is suitably installed. It provides a continuous band of attenuation over the frequency spectrum. with peaks 1d of attenuation in harmonic relation whereby proper design causes known frequencies in the exhaust to sufier maximum attenuation.

I claim:

i. A silencing device comprising a main sound 20? conductingv channel, and a closed cylindrical sidebranch and a closed conical sidebranch. acoustically coupled to said channel at points spaced along its length.

2. A silencing device comprising a closed cyas lindrical sidebranch and a closed conical sidewhich is acoustically coupled-to said sound conso ducting channel at substantially different points along the length thereof.

4. A silencing'device in accordance with claim 3 wherein the closed cylindrical sidebranch and the closed conical sidebranch are of substantially 45 equal length.

5. A silencing device according to claim 3 wherein the closed cylindrical sidebranch is coupled to said sound conducting channel at a point adjacent the eccentrically disposed outlet openso ing and. wherein the closed conical sidebranch is coupled to said channel at a point intermediate the length thereof.

6. A silencingdevice comprising a conduit, a cylindrical sidebranch open at one end and lo- 55 cated within the conduit so as to form an annular conducting channel between it and the conduit, said cylindrical sidebranch being open at one end and closed at the other and having its open end opening towards one end of the 60 conduit, and a closed conicalsidebranch located within the conduit so as to form an annular conducting channel between it and the conduit, said conical sidebranch being spaced along the axis of the conduit with respect to the cylindrias cal sidebranch, and having an open base opening towards but spaced from the closed end of the cylindrical sidebranch.

closed cylindrical sidebranch having one open end, and a closed conical sidebranch having its base open, said sidebranches being arranged c0- axially within the conduit so as to form an annular acoustic channel between them and the conduit.

9. A silencing device comprising a conduit, 9. closed cylindrical sidebranch having one open end, and a closed conical sidebranch having its base open, said sidebranches being of substantially equal acoustic length and being arranged coaxially within the conduit so as to form an annular acoustic channel between them and the conduit.

coaxially within the conduit so as to form an annular acoustic channel between them and the conduit with the open ends of the sidebranches coupled to the acoustic channel at points spaced m apart along its length.

ROLAND B. BOURNE. 

